"Built in redundancy" means that in the event of one route to a server being affected by an outage e.g. cable failure or severe weather then their other routes handle the traffic and the routing information is updated automatically. The routing system is never being used at full capacity so there is always that redundancy to allow for outages on any part of the network.
It's like having a spare bedroom available for unexpected guests - it's kept with the bed made up and aired, and not stuffed full of boxes of junk and xmas decorations so you can't get in the door!
